Summary: Age influence in telomere length and telomerase expression in peripheral blood T and B lymphocytes. We have completed an analysis of age effects on telomere length and telomerase expression in peripheral blood lymphocytes from 121 normal individuals aged from newborn to 94 years. We found: 1) Telomere shortening was observed in CD4+ and CD8+ T cells and B cells with age. However, the rate of telomere loss was significantly different in these populations, 35, 26, and 19 bp/year for CD4+, CD8+ T and B cells, respectively. In addition, CD4+ T cells had the longest average telomeres at all ages, followed by B cells, with CD8+ T cell telomeres the shortest, suggesting that these lymphocyte populations may have different replicative histories in vivo. 2) Telomerase activity in freshly isolated T and B cells was indistinguishably low to undetectable at all ages, but was markedly increased after antigen- and costimulatory receptors mediated stimulation in vitro. Furthermore, age did not alter the magnitude of telomerase activity induced after stimulation of T or B lymphocytes through antigen and costimulatory receptors or in response to PMA plus ionomycin treatment. 3) The levels of telomerase activity induced by in vitro stimulation varied among individual donors but were highly correlated with the outcome of telomere length change in CD4+ T cells after antigen receptor mediated activation. Together, these results indicate that rates of age-associated loss of telomere length in vivo in peripheral blood lymphocytes is specific to T and B cell subsets and that age does not significantly alter the capacity for telomerase induction in lymphocytes. Identification of mechanisms that regulate telomerase activity in human lymphocytes. We have analyzed expression of hTERT in T and B lymphocytes during development, differentiation, and activation . We found that hTERT mRNA and protein are expressed at detectable levels in all subsets of human lymphocytes isolated from thymus, tonsils and peripheral blood. Furthermore, telomerase activity is not strictly correlated with the levels of hTERT mRNA or protein in these ex vivo T lymphocytes. The absence of correlation between telomerase activity and hTERT mRNA could not be attributed to the presence of hTERT splice variants. Although telomerase activation in CD4+ T cells after anti-CD3 stimulation alone did not result in a net increase of hTERT protein, a parallel increase in telomerase activity and the levels of hTERT mRNA and protein was observed after anti-CD3 plus anti-CD28 stimulation in vitro, and in tonsil B cell subsets. These results indicate that expression of hTERT mRNA and protein is not sufficient to determine telomerase activity in lymphocytes, and suggest post-translational mechanisms may play role in regulating telomerase activity in lymphocytes. Here, we demonstrated two novel events associated with telomerase activation in T lymphocytes: phosphorylation and nuclear translocation of hTERT protein. Both events are post-translational and correlated with telomerase activation in lymphocytes. These results suggest that human T lymphocytes employ novel mechanisms, different from previous characterized events, in regulating telomerase activity.